Sticky back to top/contents link causes footer to jump on transition

[andysellick]

On pages such as this one when the page is scrolled near the bottom and the 'Contents' link switches between sticky and non-sticky states, this causes the content beneath the footer to 'jump' upwards.

(thanks to @fofr for the video)

The cause of this could be that when the Contents link is changed to sticky, it leaves a gap in the page so the rest of the content has to move up to fill it. This was an issue in the prototype of a sticky header (not implemented), the solution was to wrap the element in a fixed height container that wouldn't collapse.